The present invention relates to consolidation of metallic and nonmetallic powders and combinations thereof to form a consolidated body of predetermined density.
Consolidated, or high density as a percent of theoretical density, ceramic-containing bodies are useful in the cutting, drilling and shaping of hard materials, such as rock and superalloys. Additionally, these bodies possess superior hardness, strength and wear resistance and are useful in numerous applications where such properties are desirable.
Several methods are known for consolidating powders. Typically, these methods involve the formation of a preform by cold pressing the powder to be consolidated, or, alternatively, the powder to be consolidated is used to fill a can which is then sealed hermetically. Consolidation often requires pressure which is typically provided by either mechanical means, e.g., a forging press, or gaseous means, e.g., the preform is subjected to a gas at superatmospheric pressure.
Each method has problems which are unique to it. For example, U.S. Pat. No. 4,446,100 teaches a process for the consolidation of preforms using inert gas pressure. In this process, a preform is placed into a vessel which is open at the top, the preform then being embedded in glass powder. The vessel consists of graphite and is internally provided with a release layer of boron nitride. One or more vessels are placed in a high pressure furnace, wherein the preforms are consolidated. Said patent teaches that it is essential that the molten glass-containing vessel is subjected to pressure by a gaseous pressure medium and not by a piston in a mold cavity, as the glass would tend to penetrate out between the piston and the mold cavity when subjected to pressure.
A piston can be used as the pressure source when using a vessel with a closed top. For example, U.S. Pat. No. 4,428,906 teaches the consolidation of powder either as a preform or as a powder disposed within a sealed container. The material to be consolidated is encapsulated in a special monolithic pressure-transmitting medium which in turn is placed within a pot die of a press, and consolidation is performed using the ram of the press. The pressure-transmitting medium is a mixture of a fluidizing material, e.g., glass, and a rigid interconnected ceramic skeleton structure which is collapsible in response to a predetermined force. The glass is supported by and retained within the skeleton structure. As external pressure is applied by the ram, the ceramic skeleton structure collapses to produce a composite of ceramic skeleton structure fragments dispersed in a fluidizing glass. In this process, a preform is prepared for consolidation by casting around the preform a solid vessel of the composite pressure-transmitting medium. Said patent teaches that glass from the pressure-transmitting medium fills the gap between the ram and the pot die and is cooled by the metal of the ram and die, which has a high coefficient of heat transfer. The cooled glass forms a seal which prevents the glass from extruding through the gap between the ram and the pot die, and thus allows the ram to pressurize the contents of the vessel. However, this method does not use a vessel with an open top, and the one-piece vessel comprising the cast composite pressure-transmitting medium surrounding the now consolidated part must literally be hammered to fragments to recover the product. Hammering is a timehonored method of recovering consolidated bodies. See, for example, U.S. Pat. No. 3,469,976; U.S. Pat. No. 4,428,906; and U.S. Pat. No. 3,455,682. Unfortunately, the forces employed in breaking the solid medium frequently damage brittle, consolidated bodies, such as, for example, ceramic bodies, which tend to crack along with the solidified encapsulating material. Thus, the method of U.S. Pat. No. 4,428,906 solves the problem of glass extrusion but raises the problem of how to recover brittle products.
One solution to the recovery problem is to simply remove glass or other pressure-transmitting media while still molten. See U.S. Pat. No. 4,478,626. Said patent teaches the use of vessels with open tops and the use of gas pressure, and is thus similar to U.S. Pat. No. 4,446,100. However, this method is not suitable for use with a one-piece vessel, e.g., a single encapsulating mass as taught in U.S. Pat. No. 4,428,906, which is one encapsulating mass and which must be solidified to be recovered.
In summary, the prior art requires the use of sealed, deformable containers when using piston-induced pressure. The methods of the prior art can use vessels with open tops when pressure is supplied by a pressurized gas. Consolidation using pressurized gas as the pressure source is disadvantageous in that it requires long cycle times, is expensive in that it is capital intensive, and is somewhat hazardous due to the fact that it employs large amounts of energy stored as compressed gas. The use of sealed containers is disadvantageous due to the fact that a sealed container must be opened, adding additional process steps, and often leading to damage of the consolidated product. Accordingly, it would be desirable to have a process which would not require gaseous pressure, which would not require sealed containers to enclose the molten pressure-transmitting medium, and which would allow easy recovery of brittle, consolidated bodies free of structural damage.